1. Field of the Invention
The present invention relates to a digital communication system, and more particularly to a channel estimation apparatus, which estimates multi-path for a received signal by means of a synchronizing signal, that is, a signal reserved in a digital transmission/reception communication system and method thereof. The present application is based on Korean Patent Application No. 2003-10157, which is incorporated herein by reference.
2. Description of the Prior Art
As generally known in the art, in the communication between a transmitter and a receiver, a digital communication system typically inserts a synchronizing signal reserved in advance into user data to be transmitted for synchronization and equalization purposes. Generally, a PN (pseudo-noise) sequence has been used as the reserved synchronizing signal. Hereinafter, the PN sequence is described as an example of the reserved synchronizing signal.
FIG. 1 is a schematic block diagram regarding a typical channel estimation apparatus.
The channel estimation apparatus includes a correlation unit 111 and an estimating unit 113. The correlation unit 111 calculates a correlation value for correlation between a PN sequence received via a channel and a reference PN sequence. The estimating unit 113 searches a correlation value above a predetermined threshold value, and estimates the correlation value and location as the size and location of multi-path.
In this case, the predetermined threshold value should be larger than maximum correlation noise. The correlation noise is noise which has a small value due to the correlation even in a state where there is no influence by noise and adjacent data, as shown in FIGS. 2A to 2C. The correlation noise has different values according to the length of the PN sequence, as shown in Table 1.
TABLE 1length of PN sequence63255511Maximum correlation0.1109(−19.1)0.0708(−23.0)0.0412(−27.7)noiseLinear scale (dB scale)
Since the maximum correlation noise according to the length of the PN sequence is as shown in Table 1, the predetermined threshold value should be larger than the values shown in Table 1. Further, since values smaller than the threshold value are abandoned, multi-path below the threshold value included in the correlation noise cannot be estimated.
FIG. 2 is a graph showing a channel estimation performance when weak multi-path exists. The correlation unit 111 calculates a correlation value of a received signal having sizes such as 0 dB, −10 dB, −25 dB and delay such as 0Ts, 30Ts, 60Ts as shown in FIG. 2B, by means of a correlation between a PN sequence received via a channel shown in FIG. 2A and a PN sequence, which is a reference signal. Then, the estimating unit 113 estimates the channel as shown in FIG. 2C by applying a threshold value to the correlation value of the received signal as shown in FIG. 2B.
Herein, when the received channel shown in FIG. 2A is compared with the estimated channel shown in FIG. 2C, the −25 dB path of the received channel is not estimated.
FIG. 3 is a graph showing a channel estimation performance when strong multi-path exists. The correlation unit 111 calculates a correlation value of received signal having sizes such as 0 dB, −1 dB, −3 dB and delay such as 0Ts, 30Ts, 60Ts as shown in FIG. 3B, by means of a correlation between a PN sequence received via a channel shown in FIG. 3A and a PN sequence which is a reference signal. Then, the estimating unit 113 estimates the channel as shown in FIG. 3C by applying a threshold value to the correlation value of the received signal as shown in FIG. 3B.
Herein, regarding the correlation value of the received signal shown in FIG. 3B, the entire correlation noise increases due to the strong multi-path correlation noise. Further, many changes occur in peak values appearing on the multi-path. Accordingly, the threshold value set by the estimating unit 113 should increase, but if so, since multi-path below the threshold value can not be estimated, the errors of the estimated channel become large as shown in FIG. 3C.
As described above, channel estimation error generally occurs as a result of noise from correlation between a signal received via a channel and a PN sequence, inter symbol interference, and the correlation noise, etc. In this case, channel estimation error caused by the noise and inter symbol interference may be removed by means of several symbol averages, but performance deterioration caused by the correlation noise can not be removed by means of the several symbol averages.
Accordingly, there has been a problem in that channel estimation error occurs as a result of the correlation noise.